I do western blot regularly to check expression of protein X in 6 different cell lines. Now I've come across a problem with one cell line. When I run w.b. with this cell line's new cell lysate, I see different expression level (higher or lower) compared with my last result. I have no problem with the other 5 samples, but only this one.

What is the problem? Why this cell line's protein expression is not consistent with its last lysate?

p.s. I do not use any treatment. The origin of these cells are the same (colorectal). the passage and confluency is the same. the antibodies are the same, too.